Worker: HL 0.8-1.12, HW 0.8-1.28, CI 100-114, SL 0.8-1.16, SI 91-100, ML 1.08-1.76. A mediumsized, relatively robust species (HL 0.8-1.12, HW 0.8-1.28) with relatively short antennal scapes (SI 91-100) that surpass the posterior corners of the head by at least their maximum diameter. Head broader than long in most workers, broad as long in some small workers (CI 100-114), gradually tapering anteriorly behind the eyes, more strongly tapering between the eyes and mandibular insertions. Posterior margin of head slightly to moderately concave. Preocular carinae short, traversing about half the distance between the eye and the frontal carinae. In full-face view, frontal lobes simple, rounded or subtriangular, more or less symmetrical in shape (anterior side sometimes slightly longer than posterior). Anterolateral promesonotal teeth short, thick, usually pointed, not blunt. Propodeal teeth sharply pointed, shorter than the distance between their bases. Dorsal surface of body moderately tuberculate, but tuberculi are generally small and their setae short and strongly recurved. Side of mesosoma sparsely tuberculate, tuberculi very small, scarcely visible. Color variable, ranging from brownish yellow to medium reddish-brown.

Trachymyrmex desertorumHNS is broadly sympatric throughout much of southern Arizona with the similar T carinatusHNS and T pomonaeHNS (see distribution maps), but it generally occurs at lower elevations in true desert habitats, rather than in mid elevation woodlands or forests. Females can be distinguished from those of T. carinatusHNS by head shape (square to longer than broad in T. carinatusHNS ), short preocular carinae that do not closely approach the frontal carinae (closely approaching the frontal carinae in T. carinatusHNS ), and shorter antennal scapes. It may be distinguished from T. pomonaeHNS by its slightly asymmetric frontal lobes (lobes strongly asymmetric in T. pomonaeHNS ). In the field this ant is most likely to be confused with small workers of Acromyrmex versicolor PergandeHNS , which is common in many of the same habitats. In AcromyrmexHNS the head is cordate, the mesosoma is spinose, rather than tuberculate, and the frontal lobes have two short laterallydirected teeth. None of these characters is present in T. desertorumHNS .

Etymology

Wheeler collected the T. desertorumHNS types a few hundred meters from the Carnegie Desert Botanical Laboratory in Tucson, Arizona, on the bank of a dry arroyo that skirts Tumamoc hill in the "…feeble shade of the ParkinsoniaHNS and AcaciaHNS trees in the very hard, pebbly, desert soil…" (Wheeler 1910, p.100). This typical Sonoran Desert habitat no doubt inspired the species name.

Biology

Trachymyrmex desertorumHNS is a Sonoran Desert species occurring at 530-840 m elevation in central and southern Arizona and the Mexican State of Sonora. A TrachymyrmexHNS record from Willacy County, Texas (coll. Creighton 8-XI-1951) is erroneously cited as T. desertorumHNS in the literature (Wheeler & Wheeler 1985; O 'Keefe et al. 2000). The specimen belongs to T. turrifexHNS and is currently deposited in the Jeanette Wheeler Collection at the University of Arizona. Therefore, there is no evidence that T. desertorumHNS occurs in west Texas.

Typically, T. desertorumHNS occurs in Sonoran Desert habitats with palo verde ( ParkinsoniaHNS spp.), creosote bush ( Larrea tridentataHNS ), AcaciaHNS , jojoba ( Simmondsia chinensis ), honey mesquite ( Prosopis glandulosa ), and saguaro ( Carnegiea gigantea ). Nests are often in the shade under palo verde or mesquite trees, and are sometimes common in small washes. Foragers have been observed to collect green leaflets and fresh flower petals, but they have not been observed climbing plants and cutting live vegetation (C. Rabeling, personal observation). Nest craters are moderate in size (10-20 cm in diameter), conical in shape, and can be confused with the small craters of incipient A. versicolorHNS nests. Trachymyrmex desertorumHNS nests in very rocky soil, such that the limited excavatable space between the boulders often results in amorphously shaped fungus chambers. Nests contain 1 to 3 chambers that are placed up to 120 cm below the surface. Mating flights occur near dawn on mornings following summer rains. The single flight observed to date (J. Weser, pers. comm.) occurred on the same day as a mating flight of A. versicolorHNS .